The present invention pertains to a telecommunications central processor unit (CPU) access to external devices and more particularly to an arrangement for determining CPU accesses to invalid external devices by the CPU's operating software and distinguishing these invalid accesses from true hardware faults.
In telecommunications switching systems, there is generally extensive duplication of equipment, due to the public policy of providing uninterrupted communications services. These switching systems are typically controlled by one or more CPUs. Many modern CPUs provide multiple unit selection via extended memory mapping. These CPUs provide no hardware or software isolation, when invalid external devices are accessed via memory mapping.
Pure software isolation methods for invalid external device selection include, table lookups procedures. When a malfunction is detected, the unit number is examined against all possible valid unit numbers. This consumes much CPU real time. Also, it is wasteful of memory because a table of valid unit numbers must be stored to compare against the suspect invalid unit number.
In another solution, Read Only Memory (ROM) may be utilized. However, any addition of external devices to the system necessitates changing to a new ROM. This is inefficient and expensive.
Accordingly, it is the object of the present invention to provide a validity checking arrangement for extended memory mapping of external devices which is memory size efficient, easily changeable and quick operating in terms of real time.